Calcium is intimately involved in the regulation of many biological processes, and consequently organisms have evolved efficient mechanisms for its uptake, and for control of its extra- and intra-cellular levels. However, little is known about the structural basis for this regulation, though a general model for calcium-modulated protein structures, the EF-hand concept, has been put forward by Kretsinger. The major objective of this proposal is to complete X-ray crystallographic structure determination of a vitamin D-dependent calcium binding protein isolated from bovine intestine, which is involved in some, as yet uncertain, way in calcium transport across the epithelial membrane. This protein is predicted to contain an EF-hand; determination of its structure would provide the first structural test of the EF-hand concept. Excellent crystals have been obtained, and one very promising heavy atom derivative. A second, subsidiary objective is to study calcium and lanthanide binding to the bovine and the related, but larger, chicken intestinal calcium binding protein, using such spectral techniques as lanthanide fluorescence enhancement, protein fluorescence, circular dichroism and if appropriate, ultracentrifugation and rapid kinetics. There are circumstantial grounds for supposing that calcium (and lanthanides) may bind to certain polypeptide hormones; this has never been examined experimentally, and will be studied by the same techniques. A third, also subsidiary objective is to attempt crystallization of the chicken intestinal calcium binding protein, and of some large pituitary polypeptide hormones; if successful, X-ray crystallographic studies will be initiated.